For a high-power amplifier used in a terrestrial digital transmitter, a combiner having a high degree of coupling, such as a 3 dB combiner, is used. When this type of combiner is produced using a printed board, the combiner is of a broadside coupling type utilizing a microstrip line.
However, in the broadside coupling type combiner, the effective relative permittivity of an odd mode is larger than that of an even mode, and the phase velocity of the odd mode is lower than that of the even mode. Therefore, a circuit impedance Z, an even mode impedance Ze and an odd mode impedance Zo do not satisfy the relationship expressed by Z2=ZeZo in a coupler structure. The coupler structure not satisfying the relationship of Z2=ZeZo increases a return loss, and degrades isolation.